(1) Field of the Invention
The present invention relates to an aqueous suspension of composite particles and a process for producing the same. More particularly, the present invention relates to a suspension of composite particles containing core solid particles or core liquid droplets in an aqueous medium in which the solid particles or liquid droplets-containing composite particles are suspended with a high storage stability, and even when the composite particles are precipitated, they can be easily re-suspended, and a process for producing the same.
(2) Description of the Related Art
It is known that an aqueous suspension of solid particles, for example, particulate agricultural chemicals, or liquid droplets, for example, an oil, can be prepared by the following methods.
For example, Japanese Examined Patent Publication No. 46-20,519 discloses a method in which a hydrophobic agricultural chemical is wet-pulverized in water or a hydrophilic medium and the resultant hydrophilicized fine particles are suspended in the medium. Japanese Examined Patent Publication No. 58-2,440 discloses an aqueous suspension of an agricultural chemical comprising a solid agricultural chemical having a poor solubility in water or a combination of a solid agricultural chemical having a poor solubility in water with a water-soluble solid agricultural chemical; a surfactant, a water-soluble high molecular compound and water, and having a viscosity of 200 to 500 cP at a temperature of 20.degree. C.
Japanese Unexamined Patent Publication No. 57-58,601 discloses a suspension agricultural chemical comprising an agricultural chemical insoluble or scant soluble in water or organic solvents, a surfactant, a xanthane gum and water.
Japanese Unexamined Patent Publication No. 2-111,703 discloses a suspended agricultural chemical comprising a water-insoluble or scant soluble agricultural chemical, a surfactant, a water-soluble biopolymer and water.
Japanese Examined Patent Publication Nos. 7-47,521, 7-47,522, 64-7,041, 63-58,802, and 6-78,202 disclose aqueous suspension agricultural chemicals comprising a herbicide or biocide chemical and a surfactant.
Japanese Unexamined Patent Publications No. 5-201,801, No. 6-9,302 and No. 7-157,401 disclose an aqueous suspension agricultural chemical comprising a herbicide-active component or an other agricultural chemical component and a surfactant, and at least one member selected from thickeners, salts, clay minerals and metal oxide gels.
Japanese Unexamined Patent Publication No. 58-162,504 discloses an aqueous herbicide emulsion containing a liquid herbicide and a surfactant.
Japanese Unexamined Patent Publication No. 6-316,502 discloses a method of suspending a water stint soluble, solid, physiologically active substance in a liquid medium containing a nonionic surfactant having an HLB of 1 to 9 and a nonionic surfactant having an HLB of 10 to 20, the liquid medium optionally containing a clay mineral such as bentonite.
It is also known that where an aqueous suspension of solid or liquid particles is prepared, the storage stability of the aqueous suspension is variable depending on the average particle size of the particles. Also, it is believed that unless the substance to be suspended is divided to an extent such that the resultant average particle size is in the range of from 0.5 to 5 .mu.m, it is difficult to stably suspend the pulverized substance particles in a suspension medium. Therefore, to enhance the stability of the particle suspension, the substance must be finely pulverized, and thus for the fine pulverization, a specific pulverizing machine, for example, Dyno mill, air mill, hammer mill, tower mill, vibration mill or sand mill must be employed. This necessity causes the suspension cost to be high. Also, the higher the content of the particles in the suspension, the higher the viscosity of the suspension. Therefore, an aqueous suspension of fine particles having an average particle size of 0.5 to 5 .mu.m exhibits a high viscosity and thus is difficult to practically use the suspension. Namely, in the preparation of a suspension of solid or liquid particles which are insoluble or stint soluble in water, practically there is an upper limit to the content of the particles. Further, since the finely divided solid or liquid particles have a large surface area and thus the dissolution rate in water or hydrolysis rate increases, the suspension stability of the particles decreases in response to an increase in the surface area of the particles. Accordingly, in the conventional technique, an aqueous suspension of the solid or liquid particles capable of satisfying all of the requirements of high dispersion stability over a long period, high restriction of hydrolysis of the particles and a high re-suspending property of the particles has not yet been obtained.
Further, in fields of pigments, medicines, cosmetics, dyestuffs, paints and building materials which may be solid or liquid, when they are used in the state of a suspension in an aqueous medium, or the aqueous suspension is used as a starting material, it is strongly required that the aqueous suspension exhibits a high storage stability over a long time and even where a portion of the particles is precipitated during long time storage, the precipitated particle portion can be easily re-suspended. However, the aqueous suspension capable of satisfying all of the requirement has not yet been obtained.